Speciation and bioavailability of heavy metals in pyrolytic biochar of swine and goat manures

Abstract

The analysis was performed to research the speciation and bioavailability of heavy metals in manures and the biochars obtained from pyrolysis of swine and goat manures. Manures were pyrolyzed at different temperatures (200–800 °C). The concentrations of heavy metals in biochars were found to be higher than those in manures. The remaining ratios of heavy metals in biochar decreased with increasing pyrolysis temperature. Our results demonstrated that heavy metal speciation in biochar was strongly impacted by pyrolysis temperature. The bioavailable heavy metal fractions (acid soluble/exchangeable fraction and reducible fraction) were transformed into the relatively stable fraction (oxidizable fraction) and very stable fraction (residual fraction) during pyrolysis. Moreover, the data of diethylenetriamine pentaacetic acid (DTPA), HCl extraction, and toxicity characteristic leaching procedure (TCLP) in biochar indicated that bioavailability and toxicity of heavy metals in biochar were greatly reduced after pyrolysis. In summary, the pyrolysis process at 500 °C can provide a practical way to decline the bioavailability of heavy metals in manure. Biochar obtained from manure pyrolyzed at 800 °C may minimize the direct and potential heavy metal toxicities of biochar to the environment.